The present invention refers to a device and process for production of fibrous starch materials particularly destined for use in the production of paper and cardboard.
It is known that if aqueous colloid dispersions of starch in typical concentrations of between 5 and 40% by weight of anhydrous solid, is brought into contact with non-solvents (for example a saline solution of ammonium sulphate), it coagulates forming flakes of gel.
U.S. Pat. No. 4,205,025 describes a process for the production of fibrils used as paper pulp using film forming polymers including substantially water-soluble starches. By the term "fibrils", materials showing a hybrid morphology which is between a film and a fiber are intended. The film forming polymer is dissolved in water to form a solution which is then injected into a precipitating means, preferably an organic non-solvent, such as an alcohol or a ketone, with the application of shearing stress in order to obtain the formation of fibrils which are then rendered more hydrophobic through subsequent treatment in an insolublising agent.
U.S. Pat. No. 4,340,442 describes a process for the formation of fibrils which, in order to improve the hydrophobic properties of the fibrils, uses starch insoluble in water having a high amylose content (50-80% by weight), which is coagulated in a saline solution, in particular ammonium sulphate. Said starch which is substantially insoluble in water, requires a stage in which it is dissolved in alkaline solution which causes problems in the coagulation stage and problems with respect to disposal of sulphates different from ammonium salts, which are formed in said stage.
U.S. Pat. No. 4,139,699 describes a process for the production of a product having starch fiber morphology, through extrusion of a colloidal starch dispersion having a high amylopectin content in a coagulating agent. In the case where a starch having a amylopectin content of less than approximately 95% is used, it is necessary to chemically modify the starch to ensure the colloidal dispersion thereof in the aqueous system or, alternatively, the starch must be dissolved in the presence of alkaline hydroxides.
The use of alkaline hydroxides, particularly sodium hydroxide, makes the industrial application of the process described difficult, in that the coagulation stage carried out using ammonium sulphate results in the production of ammonia and formation of large quantities of sodium sulphate preventing coagulation and causing problems with respect to disposal.
U.S. Pat. No. 4,243,480 describes a process that uses the product obtained according to the process described in U.S. Pat. No. 4,139,699, for the production of paper or cardboard according to conventional paper making technology. Said product has a short fiber morphology having a diameter of between 10 and 500 microns and a length of between 0.1 and 3 mm, obtained by extruding the starch dispersion via a die into a moving coagulation bath.
U.S. Pat. No. 4,853,168 describes a process of the type described in U.S. Pat. No. 4,139,699, in which the colloidal starch dispersion adapted to be extruded is obtained by cooking an aqueous starch dispersion containing the coagulating saline solution.
In the above cited patent literature and in practical experimentation, various known devices can be used in order to finely break down the starch solution or dispersion and therefore favour a close contact with the coagulating agent, such as atomization nozzles, ejectors, mixers with stirrers, spinnerets or syringes. It has however been demonstrated experimentally that the type of device used strongly influences the final coagulated product and its properties. Devices in which the starch is coagulated in highly turbulent conditions (such as ejectors) or in which there is no ordered speed profile (mixers with stirrers), do not give rise to products with a fibrous structure, but somewhat provoke a fragmentation of the starch, with formation of flat scales (rolled onto each other) or a three dimensional aggregate.
The dimensions of these non fibrous products vary with the operating conditions and influence the characteristics of them. In the production process very small particles are lost during the separation and slow down the filtering operation in that they block the cloth; if used in the production of paper, they are not retained on the flat cloth with consequential loss of starch in the paper and an increase of COD in the paper factory waste water. On the other hand, very large particles do not integrate with the cellulose matrix fibers giving rise to defects in the produced paper.
Other negative aspect, verified for fibrids obtained from the previously described processes, consists of rather high water retention and solubility values.
A further product obtained from starch by coagulation processes, but having a fiber morphology, partly reduces the above listed disadvantages in that, thanks to its fibrous structure, it increases its compatibility with the cellulose fibers, reduces the water retention in that it is more easily filterable and reduces its solubility as it has a lower specific surface.
It would therefore be desirable to have a production of a product having fiber morphology, with dimension, size distribution and physical chemical properties such to be suitable for the production of paper and cardboard and in addition to be obtainable from low cost starch such as starch from maize or potato without adopting alkaline solutions of starch for the starch used.